ADP-ribosylation factor domain protein 1 (ARD1) initially cloned in the laboratory, differs from other ARFs by the presence of a 46-kDa N-terminal extension (p5), which acts as a GTPase-activating protein (GAP) for its C-terminal ARF domain (p3). Similar to ARF GAPs, the GAP domain of ARD1 contains a zinc-finger motif and arginine residues that are critical for activity. It differs from other ARF GAPs in its covalent association with the GTP-binding domain and the specificity of its GAP activity for the ARF domain of ARD1. The possibility of an alternatively spliced mRNA and protein product is under investigation. ARFs are presumed to play a key role in the formation of intracellular transport vesicles and in their movement from one compartment to another. Both overexpressed and endogenous ARD1 were associated with Golgi and lysosomal membranes, consistent with a role in the formation or function of lysosomes and in protein trafficking between Golgi and lysosomes. More recent data suggest a role for ARD in peroxisome function. ARD1 appears to be activated specifically by cytohesin-1, and residues responsible for specificity of the cytohesin-1/ARD1 interaction were identified. Phenotypic characterization of ARD1 "knock-out" mice, which had been progressing well, has been slowed by problems with reproduction after repeated back-crossing.